Twitter and Facebook users had highlighted the advert for a night shift worker at a store in west Suffolk on the Jobseekers' Plus website.

It was offered under the government's "sector-based work academy scheme" which is linked to payment of benefits - but Tesco said the impression that it was seeking to replace full-time workers was mistaken.

The error comes after unions called for high street chains to withdraw from government programmes that require the unemployed to work for up to six months or face losing their benefits.

Tesco has explained that the advert was "a mistake caused by an IT error by Jobcentre Plus" which was being rectified. It was an advert for work experience with a guaranteed job interview at the end of it as part of a government-led work experience scheme.

However, right to work campaigners are pressing on with the protest, at Tesco in Portcullis House, opposite the Houses of Parliament.

A spokesman for the protesters said: "Tesco reports that over the past four months some 1,400 people have worked for them without pay. Only 300 got a job with the company.

"The Tory government is slashing jobs and then punishing the jobless. And to add insult to injury, they are forcing people to work for free to boost profits for big business.

"That's why we will be demanding that workfare be scrapped immediately."

Sam James, joint national chair of Right to Work, said: "This is another example of working class people being forced to pay for a crisis created by the greed of the rich. Tesco is cashing in on people's misery. Perhaps this is what it means by 'every little helps'."

Mark Dunk of Right to Work said: "Instead of forcing people to work for free we need an end to job cuts and mass investment to address the scandal of unemployment."

A Tesco spokesman said today: "JobCentre Plus wrongly advertised a short work experience placement at Tesco as a permanent, unpaid job. This has resulted in widespread misunderstanding of our position. We are happy to re-state the facts:

"Tesco has been working in partnership with JobCentre Plus for many months to offer work experience opportunities lasting up to four weeks for young unemployed people who are struggling to find jobs.

"No one is under any obligation to take part in the scheme, and JobCentre Plus has assured us that all of those who have come to Tesco have done so as volunteers. Tesco would not take part in any mandatory scheme. This is all about helping young people who want to find a job.

"We would never offer longer term work on an unpaid basis. The Department for Work and Pensions has acknowledged that the advertisement was an error on the part of JobCentre Plus. Work experience at Tesco should, wherever possible, be a pathway to a paid job with Tesco. That has already been the case for 300 work experience participants with us so far and we hope it will be for many more people.

"We understand the concern that those who stay in the scheme longer than a week risk losing their benefits if they drop out before the end of their placement. We have suggested to DWP that, to avoid any misunderstanding about the voluntary nature of the scheme, this threat of losing benefit should be removed.

"We remain committed to offering long-term, sustainable and rewarding paths into employment for thousands of young people."

Minister for Employment Chris Grayling said: "Our work experience scheme is voluntary and thanks to companies like Tesco and many others has provided a route for literally thousands of young people to find their first job.

"The idea that providing work experience for unemployed young people is some kind of forced labour is utterly and completely absurd."

A DWP spokeswoman said: "Tesco is one of Britain's leading employers and we know the opportunities they provide are hugely valuable to young people. We know that work experience schemes make a fantastic difference to young people moving into the world of work. The scheme is voluntary and no one is forced to take part and the threat of losing the benefit only starts once a week has passed on the placement - this was designed to provide certainty to employers and the individuals taking part.
"As we made clear on Thursday, this role was incorrectly described and advertised by Jobcentre Plus; not by Tesco - there was no error whatsoever on their part."